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Fiber-Reinforced Composites - Coggle Diagram
Fiber-Reinforced Composites
Influence of fiber length
Mechanical characteristics depend on the properties of the fiber and the degree to which an applied load is transmitted to the fibres by the matrix phase
Under an applied stress, the fibre-matrix bond ceases at the fiber ends, yielding a matrix deformation pattern
Some critical fiber length is necessary for effective strengthening and stiffening of the material.
Critical length is dependent on the fiber diameter, its ultimate strength and the fiber-matrix bond strength.
When stress = ultimate strength is applied to a fiber with critical length, the maximum fiber load is achieved only at the axial centre of the fiber.
As fiber length increases, the fiber reinforcement becomes more effective
Fibers with l > lc are continuous
Fibers with l < lc are discontinuous
Influence of fiber orientation and concentration
Significant influence on the strength and other properties.
For orientation there are two extremes
Parallel alignment of the longitudinal axis of the fibres
Totally random alignment
Overall composite properties are realised when the fiber distribution is uniform.
Continuous and aligned fiber composites
Tensile-stress-strain behaviour- longitudinal loading
Properties of a composite with aligned fibres are highly anisotropic
In the uniaxial stress-strain test
Stage 1: Both fibres and matrix deform elastically
Stage 1 - 2: Proportion of the applied load borne by the fibres increases
Stage 3: Composite failure begins as the fibres begin to fracture
Composite failure is not problematic
Not all fibres fracture at the same time
Even after fibre failure, the matrix Is still intact
Elastic-Behavior - Longitudinal Loading
Assumed that the fiber-matrix interfacial bond is very good
Deformation of matrix and fibres is the same
Total load sustained by the composite = load carried by matrix phase + load carried by fiber phase
Elastic behaviour - Transverse Loading
A continuous and oriented fiber composite can be loaded in the transverse direction (90 degree angle to the direction of fiber alignment)
For this situation, the stress that the composite and both phases are exposed to is the same - is-stress state.
Longitudinal Tensile Strength
For fibre-reinforced composites that are loaded in the longitudinal direction strength is taken as the maximum stress on the stress-strain curve.
This point often corresponds to fibre fracture and marks the onset of composite failure.
This type of failure is a complex process and there are many possible failure modes
Transverse Tensile Strength
When tensile loads are present, premature failure can occur as transverse strength is usually extremely low
The reinforcing effect of the fibres is negative
Discontinuous and Aligned-Fiber Composites
Reinforcement efficient is lower for discontinuous than for discontinuous fibres
For a discontinuous and aligned fiber composite having a uniform distribution of fibres.
l>lc
l<lc
Discontinuous and Randomly-Orriented-Fiber Composites
Normally when the fiber orientation is random, short and discontinuous fibres are used
Under these circumstances, a rule-of-mixtures expression for the elastic modulus can be used
K is a fiber efficiency parameter that is usually between 0.1 - 0.6
For random-fiber reinforcement, the modulus increases with increasing volume fraction of fiber
The fiber phase
A small-diameter fiber is much stronger than the bulk material
Fibers are grouped into three classifications: whiskers, fibres and wires
Whiskers: thin single crystals that have large length-to-diameter ratios. They have a high degree of crystalline perfection and are virtually flaw-free.
Fibres: Either polycrystalline or amorphous and have small diameters
Wires: Large diameters and are used as a radial steel reinforcement.
The matrix phase
Can be a metal, polymer or ceramic
Usually metals and polymers are used because some ductility is desirable
Serves several functions
Binds the fibers together and acts as a medium that transports external stress to the fibres
Protects the individual fibers from surface damage
Seperates the fibers and prevents the propagation of brittle cracks from fiber to fiber
Essential that adhesive bonding forces between fiber and matrix are high to minimise fiber pullout.
Polymer-Matrix Composites
Consist of a polymer resin and the matrix and fibers as the reinforcement medium
Glass Fiber-Reinforced Polymer Composites
Fiberglass is a composite consisting of glass fibers (continuous or discontinuous) contained within a polymer matrix
Reasons that glass is a popular fiber reinforcement:
Easily drawn into high-strength fibers from the molten state
Readily available and can be fabricated economically using many manufacturing processes
Relatively strong as a fiber
When embedded into a plastic matrix, produces a composite with a very high specific strength.
Limitations:
Not stiff
Do not display the rigidity necessary for some applications
Limited to service temperatures < 200 C
Carbon Fibre-Reinforced Polymer Composites
Carbon is a high performance fiber material
Carbon is the most commonly used reinforcement in advanced polymer-matrix composites
Reasons:
Carbon fibers have high specific moduli and specific strengths
Retain their high tensile modulus and high strength at elevated temperatures
Not affected by moisture or a wide variety of solvents, acids and bases
Manufacturing processes are inexpensive
Aramid Fiber-Reinforced Polymer Composites
Aramid fibers are high-strength, high-modulus materials
During synthesis, the rigid molecules are aligned in the direction of the fiber axis, as liquid crystal domains
Mechanically, these fibers have longitudinal tensile strengths and tensile moduli that are higher than other polymeric fiber materials, but they are weak in compression.
Desirable properties:
high strength-to-weight ratios
Toughness
Good impact resistance
Resistance to creep
Resistance to failure
Stable to relatively high temperatures
Polymer-Matrix Materials
Most widely used and least expensive polymer resins are polyesters and vinyl esters
Metal-Matrix Composites
The matrix is a ductile metal
Can be used at higher service temperatures than their base-metal counterparts
Reinforcement can improve stiffness, specific strength, abrasion resistance, creep resistance, thermal conductivity and dimensional stability
Superalloys and alloys of Al, Mg, Ti, Cu are used as matrix materials
Reinforcement can be in the form of particulates, continuous fibres, discontinuous fibres and whiskers
Ceramic-Matrix Composites
Particulates,fibers or whiskers of one ceramic material that have been embedded into a matrix of another ceramic
The improvement in the fracture properties results from interactions between advancing cracks and dispersed phase particles
Crack initiation normally occurs with the matrix phase
Crack propagation is impeded or hindered by the particles, fibers or whiskers
Transformation toughening is a phase transformation to arrest the propagation of cracks
In general, increasing fiber content improves strength and fiber toughness
Carbon-Carbon composites
Both reinforcement and matrix are carbon
Relatively new and expensive
Desirable properties:
High tensile moduli and tensile strength
Resistance to creep
Large fracture toughness values
Low coefficients of thermal expansion
High thermal conductivities
Undesirable properties
Propensity to high-temperature oxidation
Hybrid Composites
The use of two or more different kinds of fibers in s single matrix
Have a better all-around combination of properties than composites containing only a single fiber type
The two different fibers can be combined in many ways
When they are stressed in tension, failure is usually noncatastrophic
Does not occur suddenly
Processing Of Fiber-Reinforced Composites
Pultrusion
Prepreg Production Processes
Filament Winding
